Windows version of SFS 1.5.9.8, but this bug was in previous versions too.
Bug happens again and again after some time in play. The bigger part of the solar system is exposed to the screen (more object, more distances, more time was timewarped) - the shorter this time will be, it might be even minutes.
The essence of bug is that fps in the game simply drops lower and lower very fast to absurdly low values.
If i simply press ESC, then Exit to Space Center and then immediately press Resume button at Space Center screen - i get back in game and fps always becomes very high again. No matter what is on the screen, even all solar system - very high fps anyway.
But over some time in play fps goes lower and lower, even after just 1-2 minutes sometimes, and if i didn't restore fps with going to Space Center and back - it will continue to get lower, like 15, then 10, then 8, then 5 fps. And after visiting Space Center instantly 200-500 fps, but for several minutes only.